1. Field of the Invention
The invention relates to a multilayer composite which has a barrier layer comprising a thermoplastic polyester and a protective layer comprising a material which acts as a barrier to alcohols and is selected from among a fluoropolymer and a polyolefin.
2. Description of the Related Art
In the development of multilayer composites which are used, for example, as pipes for conveying liquid or gaseous media in motor vehicles, molding compositions used have to have a sufficient chemical resistance toward the media to be conveyed and the pipes have to meet all the relevant mechanical requirements even after long-term exposure to fuels, oils or heat. Apart from the requirement of a satisfactory fuel resistance, the automobile industry demands an improved barrier action of the fuel lines in order to reduce the emissions of hydrocarbons into the environment. This has led to the development of multilayer pipe systems in which, for example, a thermoplastic polyester is used as barrier layer material. Such systems are described, for example, in EP-A-0 509 211, EP-A-0 569 681 and EP-A-1 065 048. However, these known pipe systems have the disadvantage that polyester molding compositions are susceptible to alcoholysis and hydrolysis and can lose their good mechanical strength as time goes on when alcohol-containing fuel is used.
To solve this problem, EP-A-0 637 509 proposes incorporating a fluoropolymer layer as the innermost layer to protect the polyester layer from water and alcohols. The fluoropolymer layer and the polyester layer are joined to one another by means of a bonding agent. Bonding agents disclosed are mixtures comprising firstly a fluoropolymer, flexible fluoropolymer or fluororubber and, secondly, a crystalline polyester or a polyester elastomer. In addition, a thermoplastic polyurethane, a polyamide elastomer, a modified polyolefin or a “miscibilizer” having, for example, epoxy, acid anhydride, oxazoline, isocyanate, carboxyl or amino groups can be present. However, it is not demonstrated whether this achieves any adhesion at all, apart from permanent adhesion. Due to the constituents disclosed, some of which are soluble in fuels, these bonding agents do not have a satisfactory fuel resistance. In addition, the teachings of EP-A-0 637 509 in this respect cannot readily be reproduced by a person skilled in the art on the basis of the very general information therein.
Another technical solution is described in DE-A 43 36 289. There, the polyester layer and the internal fluoropolymer layer are joined to one another by means of two successive bonding layers. However, such complex systems are expensive to produce.
As a modification thereof, a polyolefinic inner layer is also able to protect a polyester layer against the action of water and alcohol. However, there is also the problem here that satisfactory adhesion has to be achieved.
It is accordingly an object of the invention to develop a bonding agent which makes good adhesion between the polyester layer and the protective layer possible. A further object was to make adhesion which is not impaired by contact with fuel possible. Furthermore, the adhesion should be retained to a sufficient extent during the operating life of the composite. Overall, a very simple technical solution is desirable.